Various methods have been employed for offshore installation of wind turbines.
U.S. 2009/0028647 A1 discloses a method and apparatus for the mounting of offshore structures in deeper waters and suitably for mounting tall structures or relatively small width, such as a wind turbine generator. The wind turbine generator comprises a tower, a nacelle housing components and a rotor with blades. The wind turbine generator can be fully assembled on shore in its upright configuration by a tower supporting apparatus, lifted onto a transporting vessel and transferred to its final installation position using two large cranes. One limitation of this method is the use of multiple large cranes which increases costs. These large floating cranes are more suitable for calm weather operation and are subject to the environmental conditions (i.e. waves, current, etc) which will adversely affect the available window of installation.
U.S. Pat. No. 7,112,010 B1 discloses a system and method for up-righting a wind turbine assembly at an offshore installation location. Two up-righting towers along with at least one pulling string on an up-righting barge are used to assist in up-righting at least part of the wind turbine assembly. The wind turbine assembly includes a pillar, rotor and nacelle. One limitation of this method is the need for two up-righting towers to erect the windmill, which may be cumbersome and increases costs.
EP 1,581,703 A1 discloses a method of installation of wind turbines in which a nacelle and rotors are pre-assembled with a top tower section to form a pre-assembled nacelle tower at the harbour. The pre-assembled nacelle tower, a middle tower section and a lower tower section are subsequently transferred to a barge using a Turbine Roll & Lift (TRL) system. The TRL system allows for serial operation and several turbines may be loaded on the barge for a single trip from the harbour to the installation sites. The lower tower section, the middle tower section and the pre-assembled nacelle tower are separately disposed on the barge. At the installation site, the barge is docked to the foundation of the installation site. The TRL lifts a pre-assembled nacelle tower and positions it approximately above the foundation. A middle tower section is introduced under the pre-assembled nacelle tower and coupled thereto. The pre-assembled nacelle tower and the middle tower section are lifted to a greater height before a lower tower section is introduced under the middle tower section and coupled thereto. This near-completed wind turbine, which couples the nacelle tower, the middle tower section and the lower tower, is lowered into the foundation at the installation site to complete the entire installation. One limitation of this method is that the number of turbines that can be loaded onto the barge is dependent on the available jack-up vessel capacity and size of the turbines.